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Recent progresses toward the use of spin
torque oscillators in real electronics systems
Shingo TamaruSpintronics Research Center (SRC),
National Institute of Advanced Industrial Science And Technology (AIST),
Tsukuba, Japan
MIKON 2016 - 21st International Conference on
Microwaves, Radar and Wireless Communications
In Krakow, PolandMay 11th, 2016
2
Outline
1.Fundamentals of Spintronics
i. Magneto-resistance (MR) effect
ii. Spin Transfer Torque (STT) effect
2.Spin Torque Oscillator (STO)
i. Operation principle
ii. Past developments and current status of STO
i. Magnetic tunnel junction (MTJ)
ii. STO structures for higher Q factor
iii. Phase locked loop for stabilizing STO
3.Summary
3
1. Fundamentals of Spintronics
i. Magneto-resistance (MR) effect
Unti-parallel
High R
Mr
MR ratio =RP
RPRAP –(%) ,
Electrical current can be controlled by magnetization!
θ: angle b.w. Mf and Mr
Ferromagnetic layer
(Free layer)
Non-magnetic layer
(metal or insulator)
Ferromagnetic layer
(Reference layer)
Parallel
Low R
Mf
𝑅 = 𝑅0 − 𝛿𝑅 cos 𝜃
4
E
EF
E
EF
E
EF
E
EF
1. Fundamentals of Spintronics
i. Magneto-resistance (MR) effect
Density of state (DOS) diagram
High R Low R
Anti-parallel
magnetic config.
ee
Mr
Mf
Parallel
magnetic config.
ee
Mr
Mf
5
1. Fundamentals of Spintronics
i. (Giant) Magneto-resistance (GMR) effect
1
0.5
R
PRL, 61, 2472 (1988)
Prof. A. Fert
60-bilayered Fe-Cr
superlattice
About 45% drop in R
at 4.2 K (MR = 80%)
PRB, 39, 4828 (1989)
Prof. P. Grünberg1.5
Fe-Cr-Fe trilayer
MR = 1.5% at
room temp.
They were awarded 2007 Nobel prize in Physics!
6
1. Fundamentals of Spintronics
ii. Spin Transfer Torque (STT) effect
It has been shown that we can control
electrical current by magnetization.
Q: Then can’t we control
magnetization by electrical current?
A: Yes, we can!
STT effect is a reciprocal process of MR effect.
7
ee
1. Fundamentals of Spintronics
ii. Spin Transfer Torque (STT) effect
ee
e ee e
Mr
Mfe
e
e ee e
Mr
Mf
If Mf is bistable,
ee
e ee e
Mr
Mf
Uniaxial
anisotropy
Non-volatile memory
(STT-MRAM)
If Mf is monostable,Bias
field
Spin Torque Oscillator
(STO)
Free layer
is very thin,
8
JMMM, 159, L1 (1996)
Dr. J. C. Slonczewski Formulated STT as a
function of θ and P
Proposed possibility
to manipulate Mf
through STT
PRB, 54, 9353 (1996)
Prof. L. BergerPredicted STT effect
Proposed SWASER (Spin Wave Amplification
by Stimulated Emission of
Radiation)
1. Fundamentals of Spintronics
ii. Spin Transfer Torque (STT) effect
9
1. Fundamentals of Spintronics
Short summary
• We can control electrical current by magnetization.
(MR effect)
• We can control magnetization by electrical current.
(STT effect)
• These physical phenomena are utilized in,
• Spin torque oscillator (STO) • STT-Magnetic Random Access Memory (STT-MRAM)
• And many other devices…
10
2. Spin Torque Oscillator (STO)
i. Operation Principle
ee
ee
e ee e
Mr
Mf
If Mf is monostable,
Bias
field
Precession of Mf excited
by STT from DC current
I
𝑅 = 𝑅0 − 𝛿𝑅 cos 𝜃
≈ 𝑅0 − 𝛿𝑅 ∙ 𝐶 ∙ cos𝜔𝑓𝑡
where δR : R change
θ : angle b.w. Mf and Mr
C : geometrical constant
ωf : precession freq. set by
ferromagnetic resonance (FMR)
R is modulated by MR effect.
(=𝑅0 ∙ 𝑀𝑅/2)
V is given as a product of R and I.
𝑉 = 𝑅0 ∙ 𝐼 + 𝛿𝑅 ∙ 𝐶 ∙ 𝐼 ∙ cos𝜔𝑓𝑡
Microwave signal!
11
2. Spin Torque Oscillator (STO)
i. Operation Principle
ee
ee
e ee e
Mr
Mf
General features of STO
1. Very small ( d : a few 10 nm – a few 100 nm)
2. Frequency set by FMR ( Highest f : 46 GHz )
3. Ultra-Low operation voltage ( V < 500 mV )
4. Moderately low operation current ( I < 30 mA)
5. Compatible with CMOS processes I
d
12
2. Spin Torque Oscillator (STO)
ii. Past developments and current status
DC current source
Low Noise
Amp.
Bias-
Tee
Microwave
signal
Co
Cu
Co
Bias
current
Initial STO demonstration
3.6 mA
2 kOe
Pow
er
spectr
um
norm
aliz
ed
by c
urr
ent
(pW
mA
-2G
Hz
-1)
Example of STO output spectrumS. I. Kiselev, et al., Nature 425, 380 (2003).
Spectrum Analyzer
or
OscilloscopeMetallic
GMR
stack
0
2
4
13
2. Spin Torque Oscillator (STO)
ii. Past developments and current status
Problems in STO’s oscillation performance
1. Low output power
2. Wide spectral linewidth (low quality factor)
3. Multi-mode oscillation (mode hopping)
14
2. Spin Torque Oscillator (STO)
ii. Past developments and current status
Difficult to achieve high power and high Q factor simultaneously.
Breakthrough”s” needed!
15
2. Spin Torque Oscillator (STO)
ii. Past developments and current status
Metallic GMR stack
(Non-magnetic layer is a metal)Magnetic tunnel junction (MTJ) stack
(Non-magnetic layer is an insulator)
Why? – MTJ can show higher MR ratio than GMR!
This layer
is very
important.
16
2. Spin Torque Oscillator (STO)
ii. Past developments and current status
Quest for higher power = Higher MR ratio
1995 2000 2005
MR
ra
tio
at
RT
(%
)
Year
0
100
200
300
400
500
600
Amorphous Al-O
barrier
Fe(001)MgO(001)
Fully epitaxial
MTJ
Textured MTJ
FeCo(001)MgO(001)
AIST [1]
AIST [2]
IBM [3]
Crystalline
MgO(001)
barrier
Canon-Anelva
& AIST [4]
Amorphous
electrodes
FeCoBMgO(001)
Tunnel
barrier
[1] Yuasa, Jpn. J. Appl. Phys. 43, L558 (2004).
[2] Yuasa, Nature Mater. 3, 868 (2004).
[3] Parkin, Nature Mater. 3, 862 (2004).
[4] Djayaprawira, SY, APL 86, 092502 (2005).
17
2. Spin Torque Oscillator (STO)
ii. Past developments and current status
◆Benefits
・Can be deposited on various substrates (no commensurate template needed)
・Sputter deposited (good for mass production)
MgO(001)
AmorphousCoFeB
AmorphousCoFeB
Djayaprawira et al.: Appl. Phys. Lett. 86, 092502 (2005).
18
2. Spin Torque Oscillator (STO)
ii. Past developments and current status
• High power of 0.14 µW obtained. (3 orders larger than GMR-STO)
• Problems of large linewidth and multi-mode oscillation still remain.
A. M. Deac, H. Kubota et. al.
Nature Physics 4, 803 - 809 (2008).
MTJ based STO developed jointly by AIST, Osaka Univ. and Canon Anelva
CoFe
Anti-Ferromagnet
Ru
CoFeB
Cap
CoFeB
MgO
Top electrode
Bottom electrode
Structure of MTJ-STO
19
2. Spin Torque Oscillator (STO)
ii. Past developments and current status
Mf
Why is the linewidth so large?
Why does it mode-hop?
Conventional pillar shaped STO
• In-plane Mf
• Inhomogeneous internal field
• Multi-domain structure
We have to somehow get rid of effects of edge irregularities!
20
2. Spin Torque Oscillator (STO)
ii. Past developments and current status
Insulator
MR〜40%
Ta oxide
MgOLead
TaFree
Lead
50 nm
Ref.
Free
Ref.
MgO
Cap
PS
D (
µW
/GH
z)
Frequency (GHz)
2.4 µW 12 MHz
Q~330
Sombrero structure
8.64 mA
235 Oe (qH=132º)
H. Maehara, et al., Appl. Phys.
Express 6, 113005 (2013)
Solution 1: Continuous free layer (no edge)
21
2. Spin Torque Oscillator (STO)
ii. Past developments and current statusSolution 2: Perpendicularly magnetized free layer (PMF-STO)
Power : 0.55 µW
Q factor : 130
CoFeB
reference layer
FeB free layer
MgO barrier
MgO cap
• Perpendicularly magnetized free layer.
• Mf precessies w.r.t center axis.
• Uniform field distribution.
• Single domain structure.
22
2. Spin Torque Oscillator (STO)
ii. Past developments and current status
PMF-STO
We are approaching to target area
for practical applications!
Sombrero
AIST
Let’s phase lock
this STO!
23
2. Spin Torque Oscillator (STO)
ii. Past developments and current status
Ref. in
PFD
Var. in VPES
LF
HB
STO
1/48
DC
fSTO=7.344GHz
VB
PS LNA
fref=153MHz
RF out
STO: spin torque oscillator, HPF: high pass filter, LNA: low noise amplifier,
DC: down counter, PFD: phase frequency detector, LF: loop filter.
Stabilization of STO by external circuit (Phase locked loop)
24
2. Spin Torque Oscillator (STO)
ii. Past developments and current status
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
7.319 7.344 7.369Frequency (GHz)
Pow
er (
dB
m)
fspan = 50 MHz
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
7.319 7.344 7.369
Phase locked
-100
-80
-60
-40
-20
0
Frequency (GHz)7.344
Po
wer
(d
Bm
)
RBW = 1 Hz
fspan = 1 kHz
Δf<1 Hz
Free running
Δf~4.1 MHz
(Q factor ~ 1800)
RBW = 300 kHz
S. Tamaru et al., Sci. Rep. 5, 18134 (2015)
25
2. Spin Torque Oscillator (STO)
Short summary
• STO has many attractive features as a microwave
signal source in RF integrated circuits.
• Power and Q factor are steadily improving,
approaching to target area for various apps.
• STO based PLL was successfully demonstrated,
major leap for adoption in real electronics systems.
26
3. Summary
• Field of Spintronics has spawned various devices,
• Spin Torque Oscillator (STO)
• STT-MRAM
• Many others (Spin torque diode, Spin transistor, Spin
laser etc.)
• These devices have been making steady progresses,
some of them are coming close to productization.
• There is still a long way to go ahead of STO.
• Researchers in Spintronics believe a bright future is
waiting for us!
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